Operation Fruit Resilience

Guidelines to Improve Avocado Fruit Quality

1)    Introduction

The international avocado market is becoming increasingly competitive. Importers and retailers require a consistent supply of quality fruit for consumers that expect a pleasurable eating experience with every avocado purchased. If South African avocado growers are to remain competitive, there must be a renewed focus on producing high quality fruit which will withstand the rigours of the export chain even when there are logistical challenges that increase transit times.

These guidelines focus on getting the basics right to ensure that the fruit is grown correctly, harvested at the right time, and is resistant to internal disorders and decay. Each section outlines basic principles and practices with more detailed information in the Annexures and the end of the document.

2)    Plant Nutrition & Fruit Mineral Content

Basic principles

• Plant nutrition is foundational to fruit quality.
• Some nutritional elements, most notably nitrogen (N) and calcium (Ca). Play a critical role in fruit quality.
• High N and Low Ca are associated with greater susceptibility to post harvest disorders such as grey pulp and chilling injury.
• To ensure the correct levels of these and other critical elements, fertiliser programmes should be based on leaf and soil analyses.
• Effective nutrient update occurs through a healthy root system. In some cases, certain nutrient levels can be supplemented through targeted foliar sprays.

Basic principles and guidelines for plant nutrition

• Sample on a regular basis
  • Do leaf sample testing yearly on the hardened off summer flush
  • Do soil samples every second to third year
  • Results of leaf and soil analyses in conjunction with crop load, should be used to draw up a fertiliser programme.
  • Consult your fertiliser consultant or SAAGA technical staff on applications
• Know your leaf nutritional values
  • Nitrogen (N), potassium (K) and calcium (Ca). Play a critical role in fruit quality.
  • Micronutrients are just as important
  • Make corrections according to soil and leaf analysis
  • Refer to SAAGA nutritional norms
• Ensure that the timing of application of nutrients is correct
  • Incorrect timing will lead to nutritional elements not being taken up efficiently
  • Refer to SAAGA Phenological Chart for timing of applications.
• Additional management practices:
  • Ensure that there is a layer of mulch underneath trees.
  • Ensure irrigation is optimal and avoid over- and under-irrigation.

 Basic principles and guidelines for fruit mineral content

• Fruit N, K, Ca and orchard factors such as crop load influence susceptibility to grey pulp and other fruit diseases and disorders.
• Fruit mineral content samples should be done
  • Fruit samples should be taken at golf ball size (between 50 and 100g stage) – to determine fruit mineral content to avoid internal disorders after harvesting. As a minimum analyse for N, Ca, K and Zn. A full analysis would be ideal (most laboratories do only full analysis).
  • Take a random sample of at least 20 golf ball size fruit. The laboratory should he instructed to analyse the fruit flesh only. Fruit skin and pip should not be included.
  • See SAAGA nutritional norms for levels of critical elements in small fruit. Foliar sprays and/or soil applications of deficient elements should be made as soon as possible. 

Supporting documents (click to go directly to document)

1. SAAGA Nutritional norms (Annexure A)
2. SAAGA Phenological Chart (Annexure B)
3. How to take leaf and soil samples (Annexure C)

3)    Irrigation

Irrigation is crucial for fruit quality because it ensures consistent water availability, promoting optimal growth, nutrient uptake, and stress reduction in fruit crops. This, in turn, leads to improved fruit size, and overall yield, especially in regions with irregular rainfall or dry conditions. Care should be taken not to over-irrigate, as this leads to conditions conducive to Phytophthora root rot.

Basic practices

• Constantly monitor soil moisture.
  • Monitor soil moisture with probes and soil auger.
    • Use monitoring equipment to assist with irrigation scheduling to avoid over or under irrigating.
  • Know your soil types (soil classification) and manage accordingly.
  • Add mulch and cover crops to improve water holding capacity and minimise soil moisture loss through evaporation.
• Ensure optimal water supply.
  • Optimum irrigation is important, especially during critical periods such as early fruit development, when Ca accumulation in the fruit is the strongest.

Supporting documents (click to go directly to document)

1. Avocado water requirements (Annexure D)
2. Irrigation monitoring equipment – Tensiometers (Annexure E)

4)    Disease Management

Disease management is essential for avocado fruit quality because it prevents infections that can damage the fruit’s skin, flesh, and overall appearance, while also reducing the risk of post-harvest spoilage. Effective control of diseases like Cercospora (blackspot), anthracnose and root rot ensures healthier trees and fruit, leading to improved shelf life, and market value.

Basic principles and practices

• Reduce inoculum levels
  • Timing of biological and chemical sprays
  • In-field dipping directly after harvest to combat stem-end rot in the early season
  • What chemicals to use – Plant protection products list below
  • Increase fruit resistance through foliar sprays of Calcium and Silica.
• Ensure that fruit is protected when environmental conditions are suitable for fungal infection
  • Keep track of Z-factor throughout the season – Spreadsheet below
  • Once Z-values exceed “15” conditions become favourable for fungal infection
  • Anthracnose usually only develops when the fruit ripens. Infection takes place in the orchard.
• Post harvest treatment in the packhouse is critical. Ensure that dosage is correct so that MRLs are not exceeded.

What quality problems can be caused by fungal diseases?

• Anthracnose
• Cercospora & Pepper spot
• Stem-end rot

Supporting documents (click to go directly to document)

1. Control of Cercospora, tar spot, anthracnose and Stem-end rot (Annexure F)
2. Anthracnose information (Annexure G)
3. Cercospora Fact sheet (Annexure H)
4. Z-Factor formula sheet (Annexure I)
5. Stem end dip guideline (Annexure J)
6. Registered Chemicals, MRLs & PHIs (Annexure K)

5)    Harvesting Maturity Monitoring:

Testing avocado maturity before harvesting is important to ensure that the fruit will ripen properly when harvested. If harvested too early, avocados may not ripen properly or develop their characteristically creamy flavour, and are more susceptible to stem end rot. While over mature avocados can have poor shelf life and are more susceptible to developing grey pulp on ripening. Proper maturity testing can help maximize the fruit’s market value and consumer satisfaction. 

Basic principles and practices

• Monitor fruit maturity from 3 weeks before usual date of harvest
  • Fruit dry matter content is only a guide for harvesting
  • Harvesting should be based on fruit dry matter content AND ripening tests.
  • Send samples to packhouse on a weekly basis to determine maturity and monitor the trend to determine optimal harvesting.
  • Ensure your fruit meets the minimum standards set out by DALRRD (See standards attached)
  • Only harvest when all fruit are meet the minimum maturity standards

Table 4.1: Minimum Maturity to start harvesting

• There is usually significant variation in fruit maturity within a single tree and in a single orchard.
  • When sampling, take fruit throughout the orchard – large fruit, small fruit, fruit on the inside, fruit on the outside, fruit on the sunny side and fruit on the shady side
  • Stripping random trees just before harvest can help determine variation of size distribution and dry-matter content expected of that orchard.
• Ripening tests (Important for early season fruit)
  • Bench ripening protocol
    • Store at room temperature ±14 days.
    • Determine if fruit ripens acceptably (no shrivelling, or rubbery flesh)
  • Forced/Triggered ripening test
    • In a sealed container with a Ethylene pump, pump 100ppm ethylene into container for 24-36 hours – time may vary depending on container size.
    • Place fruit on bench at room temperature and after 3 days see if fruit ripens acceptably.
    • This method has not been scientifically proven – and is based on experience from a packhouse. However, it does allow fruit to ripen a lot quicker than fruit left to ripen at room temperature, which could take up to 14 days at the beginning of the season (SAAGA to conduct research to determine protocols for triggered ripening tests).
  • Trees under stress should not be harvested as soon as possible after minimum fruit maturity is reached.
  • Over mature fruit and risk of Internal Disorders
    • Avocados with dry matter content exceeding the values in Table 4.1 are at higher risk of developing internal disorders like grey pulp and should not be exported.

Table 4.2: Maximum Maturity cut off points (% fruit dry matter) (adapted from Kruger et al., 2004)

*Maluma Hass data based on Pinkerton data – Yet to be scientifically proven

**No data yet for Lamb Hass – Research currently underway

Dispensations

1. Exporters may apply for dry matter dispensations if they believe that they can deliver a quality product at reduced dry matter. The exporters’ decision should be based on ripening tests which show that the fruit will be of acceptable quality.

What quality problems can occur due to fruit being harvested immature?

1. More prone to chilling injury/cold damage if harvested early
2. Shriveling /rubbery flesh
3. Internal discoloration
4. Uneven ripening
5. Stringiness

Supporting docs (click to go directly to document)

1. Principles for avocado fruit maturity standards (Annexure L)
2. SAAGA maximum maturity guidelines (Annexure M)
3. Fruit Dry Matter Determination & Ripening Record Sheet (Annexure S)
4. Maluma Hass avocado cultivar: Cultivation, ripening and post-harvest protocols (Annexure R)

6)    Harvesting Practices:

Careful harvesting of avocados is essential to preserve fruit quality, as rough handling can cause bruising, lenticel damage, cuts, or stem damage, which accelerates decay and diminishes appearance and shelf life. Gentle harvesting techniques help maintain the avocado’s structural integrity, ensuring a smoother ripening process, and higher market value.  

Basic principles and practices

• Avoid picking wet fruit
  • Wet fruit can lead to higher susceptibility to lenticel damage, don’t pick fruit when it is raining
  • When picking in wet conditions, where soil moisture is close to field capacity – make sure fruit is dry
  • Handle fruit with care. Early season fruit is more sensitive. To avoid damage in the early season, only fill lugs/bins to three-quarter capacity
  • Lenticel damage can be an entrance point for fungal infections
• Harvesting should be done meticulously and carefully
  • Train workers and equip them with the right tools
  • Workers should handle fruit with care and avoid cuts and bruises
  • Rough handling can lead to internal damage and overall decrease in quality of fruit
  • Workers should not snap pick, especially during early season, as the pedicel protects fruit from infection
• Infield stem end dipping
  • Early season conditions are more favourable for fungal pathogens as humidity is still high
  • Growers are advised to do infield stem end dipping with registered products

Fruit should be taken to the packhouse as soon as possible after harvesting. Otherwise, store fruit in a shady and cool place until a load is ready to be sent to the packhouse.

What quality problems can be caused during harvesting?

1. Internal Bruising
2. Fungal diseases (anthracnose, stem end rot)
3. Lenticel damage

Supporting documents (click to go directly to document)

1. SAAGA maximum maturity guidelines (Annexure M)
2. Avocado harvesting critical guidelines (Annexure N)
3. Protocols for reducing lenticel damage (Annexure O)
4. Stem end dip guideline (Annexure J)

7)    Packhouse Practices:

Proper packhouse practices are critical for maintaining avocado fruit quality. 

Basic principles

• The packhouse’s main purpose is to grade, pack, cool and temporarily store fruit
  • Growers need to apply adequate measures to reduce the inoculum of fungal pathogens during the growing process
  • Ensure that fruit gets to the packhouse timeously to maintain quality
  • Packhouse needs to ensure that best quality fruit leaves the packhouse
• Fruit needs to be handled with care on pack line to avoid bruising and lenticel damage
  • Minimisethe use of roller brushes in the pack line
  • Avoid large drops on pack line
  • Check pack line and modify it if there are areas where fruit is turned and rolled unnecessarily
• Packhouses should use the correct chemicals
  • Products previously registered for use on avocados, such as prochloraz, can contaminate the fruit and lead to rejections. Ensure that the packline is properly cleaned and free from contamination. Refer to document below on cleaning of packlines.
  • Refer to SAAGA Plant protection products & MRL list on what products can be used
• Regularly sanitise the whole packhouse
  • This will reduce the inoculum level of fungal pathogens in the pack house
• Fruit must be cooled down as soon as possible
  • Start cooling packed fruit immediately after packing to the required shipping temperature
  • Where possible, build up pallets in cold rooms to ensure that partially filled pallets are not left unrefrigerated for extended periods, which could compromise fruit quality
  • See further information in Shipping Temperatures section

What quality problems can be caused in the packhouse?

1. Bruising
2. Lenticel damage
3. Cold damage

Supporting documents (click to go directly to document)

1. Registered Chemicals, MRLs & PHIs (Annexure K)
2. Protocols for reducing lenticel damage (Annexure O)
3. Protocol on cleaning of packlines to remove prochloraz residues (Annexure Q)

8)    Shipping Temperatures:

The correct cooling of avocados is vital in maintaining quality during shipment and maximising shelf-life post shipment. Shipping temperatures must be such that the fruit does not ripen in transit without developing chilling injury. Less mature avocados are more susceptible chilling injury and require higher shipping temperatures. More mature avocados can tolerate lower shipping temperatures and require lower temperatures to prevent them ripening in transit.

Basic principles and practices

• Rapid cooling after harvest
  • Start cooling immediately after packing to the required holding temperature
  • The fruit should reach the required pulp temperature (within 2-3 ºC of the set air delivery temperature) within 24 hours after packing. (Table 7.1)
  • Best practice is to build up pallet loads in a refrigerated area
• Shipping temperature depends on fruit dry matter content (maturity)
  • Less mature fruit is more susceptible to chilling injury and should be shipped at higher temperatures
  • More mature fruit is less susceptible to chilling injury and should be shipped at lower temperatures
  • Pinkerton avocados are particularly sensitive and require careful temperature management
  • Packhouses must consistently monitor fruit maturity throughout the season to ensure the shipping temperatures are adjusted appropriately for each batch
  • Ensure that the temperature monitoring equipment in cold rooms is accurate. Use regularly calibrated electronic and mercury thermometers to avoid temperature errors

Table 7.1: SAAGA suggested shipping temperatures for Avocados

What quality problems can be caused during shipping?

1. Chilling injury
2. Fruit softening in transit
3. Fruit decay

Supporting documents (click to go directly to document)

• Shipping temperature guidelines based on fruit dry matter content in the Avo Packing Guide (page 20 -22)  (Annexure P)
• Maluma Hass avocado cultivar: Cultivation, ripening and postharvest protocols (Annexure R)